Under ordinary circumstances, thermoregulatory mechanisms exist in the healthy human body to maintain the body at a constant temperature of about 37.degree. C. (98.6.degree. F.), a condition sometimes referred to as normothermia. To maintain normothermia, the thermoregulatory mechanisms act so that heat lost to the environment is replaced by the same amount of heat generated by metabolic activity in the body. For various reasons, a person may develop a body temperature that is below normal, a condition known as hypothermia.
Accidental hypothermia may result when heat loss to the environment exceeds the body's ability to produce heat internally or when a person's thermoregulatory ability has been lessened due to injury, illness or anesthesia. Accidental hypothermia is generally a dangerous condition that can have serious medical consequences. For example, hypothermia may interfere with the ability of the heart to pump blood or the ability of the blood to clot normally. Hypothermia may also interfere with various temperature sensitive enzymatic reactions in the body with resultant metabolic and biochemical consequences, and has sometimes been associated with impaired immune response and increased incidence of infection.
Simple methods for treating hypothermia have been known since very early times. Such methods include wrapping the patient in blankets, administering warm fluids by mouth, and immersing the patient in a warm water bath. If the hypothermia is not too severe, these methods may be effective. However, wrapping a patient in a blanket depends on the ability of the patient's own body to generate heat to re-warm the body. Administering warm fluids by mouth relies on the patient's ability to swallow, and is limited in the temperature of the liquid consumed, and the amount of fluid that may be administered in a limited period of time. Immersing a patient in warm water is often impractical, particularly if the patient is simultaneously undergoing surgery or some other medical procedure.
More recently, hypothermia may be treated by the application of a warming blanket that applies heat to the skin of the patient. Applying heat to the patient's skin, however, may be ineffective in providing heat to the core of the patient's body. Heat applied to the skin has to transmit through the skin by conduction or radiation which may be slow and inefficient, especially if the patient has a significant layer of fat between the warming blanket and the body's core.
Paradoxically, the application of warmth to a hypothermic patient's skin, whether by immersion in hot water or application of a warm blanket, may actually exacerbate the problem and may even induce shock. The body has certain thermoregulatory responses to cold that work to conserve heat in the body's core, specifically vasoconstriction and arterio-venous shunting (AV shunts). Vasoconstriction occurs when the capillaries and other blood vessels in the skin and extremities constrict so that most of the blood pumped by the heart circulates through the core rather than through the skin and extremities. Similarly, in AV shunting, naturally occurring blood shunts exist between some arteries providing blood to capillary beds in the skin and extremities and veins returning blood from those capillary beds. When the body is cooled, those shunts may be opened, allowing blood to by-pass those capillary beds altogether. Thus when the body is cooled, the tissues in the extremities, and particularly at the surface, have little blood flowing to them and may become quite cold relative to the body's core temperature.
When heat is applied to the skin of a hypothermic patient, the temperature sensors in the skin may cause the vasoconstriction to reverse and the AV shunts to close. When this happens, blood from the core floods into the very cold tissue on the body surface and extremities, and as a result the blood loses heat to those tissues, often far more than the amount of heat being added by the surface warming. As a result, the victim's core temperature may plummet and the patient may even go into shock.
Partly in response to the inadequacies of surface application of heat, methods have been developed for adding heat to a patient's body by internal means. A patient being administered breathing gases, for example a patient under anesthesia, may have the breathing gases warmed. This method may be effective but is limited in the amount of heat that can be administered without injuring the lungs. Similarly, a patient receiving IV fluids may have the fluids warmed, or a bolus of warm fluid may be administered intravenously. This may be effective in the case of mild hypothermia, but the temperature of the IV fluid is limited by the temperature that will be destructive to the blood, generally thought to be about 41.degree. C.-49.degree. C., and the amount of fluid that is acceptable to administer to a particular patient.
A more invasive method may be used to add heat to a patient's blood, particularly in the case of heart surgery. Blood is removed from a patient, circulated through a cardiopulmonary by-pass (CPB) system, and reintroduced into the patient's body. The blood may be heated or cooled before being reintroduced into the patient. This CPB method is both fast and effective in adding or removing heat from a patient's blood, but has the disadvantage of involving a very invasive medical procedure which requires the use of complex equipment, a team of highly skilled operators, and is generally only available in a surgical setting. It also involves mechanical pumping of blood which is generally very destructive of the blood tissue resulting in the cytotoxic and thrombolytic problems associated with removal of blood from the body, mechanical pumping of the blood, and channeling the blood through various machines and lines.
Means for adding heat to the core of the body that do not involve pumping the blood with an external, mechanical pump have been suggested. For example, a method of treating hypothermia or hypothermia by means of a heat exchange catheter placed in the bloodstream of a patient was described in U.S. Pat. No. 5,486,208 to Ginsburg, the complete disclosure of which is incorporated herein by reference. That patent discloses a method of treating or inducing hypothermia by inserting a heat exchange catheter having a heat exchange area including a balloon with heat exchange fins into the bloodstream of a patient, and circulating heat exchange fluid through the balloon while the balloon is in contact with the blood to add or remove heat from the bloodstream. (As used herein, a balloon is a structure that is readily inflated under pressure and collapsed under vacuum.) Under certain conditions heat is generated within the body or heat is added from the environment in excess of the body's ability to dissipate heat and a persons develops a condition of abnormally high body temperature, a condition known as hypothermia. Examples of this condition may result from exposure to a hot and humid environment or surroundings, overexertion, or exposure to the sun while the body's thermoregulatory mechanisms are disabled by drugs or disease. Additionally, often as a result of injury or disease, a person may establish a set point temperature that is above the normal body temperature of about 37.degree. C. The set point temperature is the temperature that the body's thermoregulatory mechanisms actto maintain. Under ordinary circumstances, this is about 37.degree. C. but in other cases, such as fever, the body may establish a different set point temperature and act to maintain that temperature.
Like hypothermia, hypothermia is a serious condition that may sometimes be fatal. In particular, hypothermia has been found to be neurodestructive, both in itself or in conjunction with other health problems such as stroke, where a body temperature in excess of normal in conjunction with a stroke or traumatic brain injury has been shown to results in dramatically worse outcome.
As with hypothermia, counter-parts to simple methods for treating the condition exist, such as cold water baths and cooling blankets, and more effective but complex and invasive means such as cooled breathing gases and blood cooled during CPB also exist. These, however, are subject to the limitations and complications as described above in connection with hypothermia. In addition, the thermoregulatory responses such as vasoconstriction, AV shunting and shivering, may act directly to combat the attempt to cool the patient and thereby defeat the effort to treat the hypothermia. This is especially true in the case of fever, where the body may establish a set point temperature higher than normothermia and actively resist efforts to reduce the body's feverish temperature to normothermia.
Although both hypothermia and hypothermia may be harmful and require treatment in some case, in other cases hypothermia, and especially hypothermia, may be therapeutic or otherwise advantageous, and therefore may be intentionally induced. For example, periods of cardiac arrest in myocardial infarction and heart surgery can produce brain damage or other nerve damage. Hypothermia is recognized in the medical community as an accepted neuroprotectant and therefore a patient is often kept in a state of induced hypothermia during cardiovascular surgery. Likewise, hypothermia is sometimes induced as a neuroprotectant during neurosurgery.
It is sometimes desirable to induce whole-body or regional hypothermia for the purpose of treating, or minimizing the adverse effects of, certain neurological diseases or disorders such as head trauma, spinal trauma and hemorrhagic or ischemic stroke. Additionally, it is sometimes desirable to induce whole-body or regional hypothermia for the purpose of facilitating or minimizing adverse effects of certain surgical or interventional procedures such as open heart surgery, aneurysm repair surgeries, endovascular aneurysm repair procedures, spinal surgeries, or other surgeries where blood flow to the brain, spinal cord or vital organs may be interrupted or compromised. Hypothermia has also been found to be advantageous to protect cardiac muscle tissue after a myocardial infarct (MI).
Neural tissue such as the brain or spinal cord, is particularly subject to damage by vascular disease processes including, but not limited to ischemic or hemorrhagic stroke, blood deprivation for any reason including cardiac arrest, intracerebral or intracranial hemorrhage or blockage, and head trauma. In each of these instances, damage to brain tissue may occur because of brain ischemia, increased intracranial pressure, edema or other processes, often resulting in a loss of cerebral function and permanent neurological deficits. Although the exact mechanism for neuroprotection is not fully understood, lowering the brain temperature is believed to effect neuroprotection through several mechanisms including, the blunting of any elevation in the concentration of neurotransmitters (e.g., glutamate) occurring after ischemic insult, reduction of cerebral metabolic rate, moderation of intracellular calcium transport/metabolism, prevention of ischemia-induced inhibitions of intracellular protein synthesis and/or reduction of free radical formation as well as other enzymatic cascades and even genetic responses. Thus intentionally induced hypothermia may prevent some of the damage to brain or other neurological tissue during surgery or as a result of stroke, intracerebral hemorrhage and trauma.
Intentionally inducing hypothermia has generally been attempted by either surface cooling or by-pass pumping. Surface cooling has generally proved to be unacceptably slow, since the body heat to be removed must be transmitted from the core to the surface, and has sometimes been altogether unsuccessful since the body's thermoregulatory mechanisms act to prevent surface cooling from reducing the core temperature of the body. For example, the vasoconstriction and AV shunting may prevent heat generated in the core from being transmitted to the surface by the blood. Thus the surface cooling may only succeed in cooling the skin and surface tissue, and not succeed in reducing the core temperature of the patient to induce a hypothermic state.
Another thermoregulatory mechanism that may thwart attempts to reduce core temperature by surface cooling is shivering. There are numerous temperature sensors on the body's surface, and these may trigger the body to begin shivering. Shivering results in the generation of a significant amount of metabolic heat, as much as five times the norm, and with the blood to the surface of the body greatly constricted, the cooling blanket can only reduce the temperature of the patient very slowly, if at all. If the patient has fever and thus an elevated set point temperature, and thus shivers at a temperature above normothermia, it has been found that cooling blankets are often unable to reduce the patient's temperature even to normothermia.
Additionally, because the heat transfer from the surface to the core of a patient by the application of cooling blankets is slow and inefficient, the control of the patient's core temperature by surface cooling is very difficult, if not impossible. The temperature of the patient tends to "overshoot" the desired low temperature, a potentially catastrophic problem when reducing the core temperature of a patient, especially to moderate or sever levels. Speedy adjustment of core temperature by surface cooling is difficult or even impossible, particularly if precise control is needed.
As is the case with the use of CPB machinery to warm blood removed from the body and replace it into the body, by-pass may be fast and control may be relatively precise, especially if large volumes of blood are being pumped through the system very quickly. However, as was previously stated, this method is complex, expensive, invasive and generally damaging to the blood, particularly if continued for any significant period of time.
Besides intentionally induced hypothermia or hypothermia, it is sometimes desirable to control a patient's temperature to maintain the patient at normothermia, that is normal body temperature of about 37.degree. C. For example, in a patient under general anesthesia, the body's normal thermoregulatory centers and mechanisms may not be fully functioning, and the anesthesiologist may wish to control the patient's body temperature by directly adding or removing heat. Similarly, a patient may lose an extraordinary amount of heat to the environment, for example, during major surgery, and the patient's unaided body may not be able to generate sufficient heat to compensate for the heat lost. This is especially true where, as a result of the anesthesia used during surgery, the patient's normal thermoregulatory response is reduced or eliminated. A device and method for controlling body temperature by adding or removing heat to maintain normothermia, would be desirable.
Additionally, a patient may suffer disease or trauma or have certain substances introduced into its body that cause an increased set point temperature resulting in fever, as in the case of infection or inflammation. The unaided body may then act to maintain a temperature above 37.degree. C. and surface cooling may be ineffective in combating the body's thermoregulatory activity and reestablishing normothermia. Where, for example in stroke, the presence of fever has been found to correlate with very negative outcome, it may be very desirable to maintain normothermia.
The mammalian body generally functions most efficiently at normothermia. Therefore maintaining hypothermia in a portion of the body such as the brain or heart while maintaining the temperature of the rest of the body at normothermia may provide for protection of the target tissue, e.g. neuroprotection of the brain or protection of the myocardium while allowing the rest of the body to function at normothermia.
For the foregoing reasons, there is a need for a means to add or remove heat from the body of a patient in an effective and efficient manner, while avoiding the inadequacies of surface heat exchange and the dangers of CPB methods that require pumping the blood from the body of the patient, heating or cooling the blood, and then returning it to the patient. There is the need for a means of rapidly, efficiently and controllably exchanging heat with the blood of a patient so the temperature of the patient or target tissue within the patient can be altered, or maintained at some target temperature.